Complexation Behavior of Heterocyclic Hydrazones. II. Effects of Steric Factors on Formation Constants for Nickel(II) Complexes with Heterocyclic Hydrazones

The equilibria of nickel(II) ion with thirteen nitrogen-heterocyclic hydrazones (neutral form: HL) with and without blocking group(s) have been investigated spectrophotometrically in 28% aqueous dioxane solution at 25 °C and an ionic strength of 0.2 (KCl). For a series of 2-pyridinecarbaldehyde 5-substituted-2-pyridylhydrazones, the following linear free energy relationships (LFER) were derived between the overall formation constants, β1, β1′, and β2, of Ni(HL)2+, NiL+, and Ni(HL)22+ complexes, respectively, and the dissociation constants, Ka1 and Ka2, of H3L2+ and H2L+, respectively: log β1 = 0.95 (pKa1 + pKa2)/2 + 4.60, 1/2 log β2 = 0.95 × (pKa1 + pKa2)/2 + 4.82, and log β1′ = 1.40 (pKa1 + pKa2)/2 + 9.73. The steric effects of the 6-methyl group in pyridine and the quinolyl ring in place of pyridyl group on the complex formation were evaluated on the basis of LFER. The complex stabilization effect of the pyridyl group substituted for formyl hydrogen was also evaluated for Ni(HL)22+, Ni(HL)2+, and NiL+ complexes. These results together with LFER were discussed in conjunction with the thermodynamic parameters for the formation of Ni(HL)2+ complex and the ligand field parameters, Dq, for Ni(HL)2+ and Ni(HL)22+ complexes.